Prostheses for use in total hip replacement typically include an elongate member for implantation in the femur. This implant member typically includes a distal tip portion for location deep within the bone, a proximal neck portion for providing articulation of the joint prosthesis and an elongate stem portion which extends from the distal tip portion to the proximal neck portion. In use, the elongate stem portion is also located within the bone to provide a foundation or support for the artificial joint and to transmit forces to the skeletal structure. The proximal neck portion of the implant projects from the end of the femur to form part of the articulated joint.
When looking at either the anterior or posterior surface of a femoral implant, the outer contour of the medial and lateral sides define the implant profile in the medio-lateral plane. When looking at either the medial or lateral surfaces of the implant, the outer contour of the anterior and posterior sides define the implant profile in the antero-posterior plane (i.e. the sagittal plane).
The stem portion of a femoral implant is typically integrally formed with the neck portion. Furthermore, the stem portion typically has a proximal end region adjacent the neck portion which, in use, is located within the proximal or metaphyseal end of the femur. The neck portion therefore joins the stem portion at this proximal or metaphyseal end region of the stem portion, and this metaphyseal end region of the stem typically has a profile in the medio-lateral plane that converges distally. That profile is typically defined by a curved or angled medial outer contour and a curved or straight lateral outer contour.
Because the implant members for hip prostheses are to be inserted into the femur, the stem portions of those implants need to be relatively narrow (particularly in their more distal parts) to fit within the intramedullary canal of the bone. This applies especially to longer implants (i.e. greater than 190 mm). Accordingly, the stem portion of an implant will typically taper in the medio-lateral plane, and possibly also in the sagittal plane, decreasing in transverse width towards the distal tip portion. In short or standard length implants, this tapering is often continuous along the length of the stem portion to the distal tip portion. In longer implants, however, in order to retain the desired structural integrity and function of the implant, the tapering will sometimes terminate at an approximately constant cross-section, which then continues to the distal tip portion of the implant.
A disadvantage of femoral implants with stem portions having a constant cross-sectional extent is that the constant cross-section part does not provide optimum transfer of loads to the surrounding bone structure in use. A further disadvantage of such implants is that they can be harder to extricate from their cemented position within the bone, in the event that the position or orientation of the implant requires revision at some later stage in the life of the prosthetic joint.
The present invention therefore aims to provide a new and improved prosthetic implant which substantially overcomes or at least ameliorates either or both of the above disadvantages.